The outburst of the Draconid meteor shower in 2018: an analysis

Draconid outburst in full progress! These Draconids were captured on 8 oktober 2018 between 22:00-00:00 UT with CAMS 351 camera (CAMS Benelux network), located at Ermelo, the Netherlands and is operated by the author. The bright star in the middle is Kochab (beta Ursa Minor).

Introduction
On the evening of October 8th, 1933, Dirk Teunissen (the now deceased father-in-law of the author and at that moment 13 years old) is cycling home from the Ambachtschool Over-Veluwe in the small city of Harderwijk to the village of Ermelo. Outside Harderwijk on what now is called the Harderwijkerweg, he sees an uncommon phenomenon. The clear sky is filled with many shooting stars. He saw them wherever he looked. Most of the shooting stars were weak and very slow. Sometimes several appeared at once! After watching this spectacle for a while, he cycles to home, wondering about the phenomenon. When he comes home he tells what he had seen to the family members who also go outside to watch. Meanwhile, the numbers are much less but it is still a beautiful sight.
Dirk witnessed the impressive Draconid meteorshower of 1933. The ZHR rose that year to 6000-10000 [9]. The parent body of the Draconids is comet 21P/Giacobini-Zinner which has a period of 6.6 years. In 1933 and 1946, the Draconids caused impressive meteorshowers. In later years more Draconid outbursts were detected but they never reached the level of 1933 and 1946. The outbursts almost always occurred in the years of perihelium passage of Comet 21P/Giacobini-Zinner, among others in 1985 when an outburst (ZHR 700) was observed above Japan, as well as in 1998 (ZHR 500). On 8 October 2011, an outburst was observed from Europe. For example, Dutch and Belgian observers observed the outburst from Denmark, Germany and Portugal [2, 3 & 4]. The ZHR reached 350 [5] that year. The period of 6.6 years of 21P is also the reason that a large outburst occurred every 13 years and smaller ones every 6 or 7 years in between. In 2005, a small outburst (ZHR 35) was observed from the Draconids, but this took place just before the end of the evening twilight in the Netherlands.
Also in the years after the bigger outbursts some low Draconid acitvity could observed: in 1999 Marco Langbroek witnessed unexpected Draconid activity with a ZHR 10-20 [6] and in 2012 the CMOR radar showed a very high activity of the Draconids, but almost all of them were very weak Draconids. In the off-season period of the Draconids, usually a few Draconids are seen, but the activity is very low/nihil.

Predictions for 2018
Several astronomers made predictions for the Draconids in 2018 [7]. Mikiya Sato found a close approach of the Earth to the dust trail of 21P/Giacobini-Zinner from 1953. Due to the 1985 approach to the Earth, the dust might have been dispersed, but even then, there should be something observable with a ZHR of 20-50 around 9 October 2018 at 00:14 UT (λꙨ 195.406). Jérémie Vaubaillon found a possible maximum on 8 October 2018 at 23:31 UT (λꙨ 195.374) with an expected ZHR of 15. Mikhail Maslov found several dust trails, but none of them came close enough to the Earth to expect high activity. He expected a ZHR of 10-15 around 23:34 UT on 8 October 2018. An overview of all the predictions can be found in table 1. This table comes from [1].
Interesting is the prediction of Egal et al. In [1] she states that the Earth moves through a hole in the denser parts of the Draconids that was struck by the passage of the Earth through the dust trail in 1985 (see figure 1).
Taken together, the expectations were therefore not very high. Nevertheless, a number of observers decided (if necessary) to travel great distances in order to be able to observe something. The reason for this was that the comet had its perihelion a month earlier on September 10, 2018. That gave some hope for higher activity than had been predicted. And they were not disappointed!

Table 1. Predictions for the Draconids 2018, table taken from [1]

Figure 2. Draconid meteoroid nodal crossings close to the Earth’s orbital plane on 8-9 of October 2018. Each symbol indicates a particles ejection epoch (legend) while the Earth’s path is shown in blue with L1 in green and L2 in red. Figure and text from [1]

8/9 October 2018
As the night falls over Europe, the already active observers observed low but clearly detectable Draconid activity. The ZHR is during the first hours around 10. Just after 21 UT the activity starts to rise. A rapid increase that continues until just before 23 UT. Most observers then report considerable activity, despite the fact that the radiant is already low in the northwestern sky. Some reports from the field:

Jure Atanakov from Slovenia: “Observed 22:40-00:44 UT under mediocre conditions, LM about 6.5 and variable cloud cover (0-40%). Peak seemed to be around 23:00-23:20 UT. Rates were probably >100/h, even with the radiant below 30 degrees. Will be surprised if peak ZHR is not around several hundred. Possible secondary peak around 00:00 UT.”

Michel Vandeputte from Belgium: “A lot of faint stuff; but also, nice events, sometimes nice very white appearances with flares: the typical ‘fragile’ Draconid. The activity remained long time stable and modest until suddenly activity started to pick up; a period when about one per minute appeared. The outburst had materialized well in advance of the predicted observing window. However, it did not remain with just short pulses. The activity increased further out of nothing, multiple meteors were seen per minute, even two or three at the same instance! Yes, this was going hard: probably getting at a ZHR of about 100, certainly taking into account the low position of the radiant! Everywhere nice long meteor trails at the sky thanks to the decreasing radiant position. Not only faint stuff, but sometimes very nice meteors up to -2, even one small fireball!”

Kai Gaarder from Germany: “A short update from me before heading for the train station: 7 hours and 15 min of observations under variable, but quite good observing conditions. 300 meteors observed, among them 191 Draconids. Clearly a good outburst from 21:00 onwards, with uncorrected hourly rates of 34, 57 and 53 the next 3 hours! This was real fun, and a big success!

When the night falls over America, high Draconid activity is visible, also thanks to the high radiant level. Well-known meteor observers such as Paul Jones, Bruce McCurdy and Pierre Martin¹ see impressive numbers of Draconids that decrease over time. And when it finally gets dark on the west coast of America (including Wesley Stone observations), the activity has virtually disappeared.

Figure 3. Five bright Draconids captured with CAMS 354 camera. All these Draconids showed fragmentation (flares). CAMS 354 camera (CAMS Benelux network) is located at Ermelo, the Netherlands and is operated by the author. The picture shows the constellation of Auriga.

Processing of the data
The author obtained a lot of data via the IMO website. The author also received a lot of data from a few observers who do not report to IMO. The ZHR curve shown on the IMO website (https://www.imo.net/members/imo_live_shower?shower=DRA&year=2018) is based on 2074 Draconids collected by 39 observers. For the determination of the population index r a large number of observations could be used, as well as for the ZHR values.

Population index r
After an extensive check of the magnitude distributions supplied by the observers, a total of 1075 Draconids remained that could be used to determine the population index r. An attempt has been made to obtain the evolution of the population index r during the period of 8 October 2018 18:00 UT until 9 October 2018 06:00 UT. For this evaluation r[0;5] was the most suitable magnitude range. This resulted in table 2. No real trend emerges from result but rather a slightly variable r value.

Table 2. Population index r Draconids 8/9 October 2018

ZHR
The ZHR was determined with a mean r value of 3.3. For this purpose, counting periods of 10-15 minutes were used and these were always calculated in overlapping periods. A total of 2763 Draconids have been used for the ZHR analysis. This resulted in figure 4.

Figure 4. ZHR graph of the Draconid outburst of 8/9 October 2018.

What is clearly noticeable in the graph is the fast but stable increase in activity to the maximum around λꙨ 195.35 (8 October 2018 just before 23 UT). After that, a fairly flat activity follows until 0:15 UT, after which the activity with two strong sub-peaks drops back to λꙨ 195.44 (9 October 01:07 UT) and λꙨ 195.48 (9 October 2018 02:09 UT). Then the ZHR drops quickly to 10. Impossible to explain what has caused these subpeaks. The predictions from table 1 were expected in the period from λꙨ195.35 to λꙨ 195.42. That is exactly the period with the 1st peak followed by the flat ZHR.
In addition, a few Draconids were observed visually as well as with CAMS in the night before (Europe) and after (Europe and US).
What is striking about this curve is that we are looking at a ZHR curve with a nice increasing wing between λꙨ 195.27 to λꙨ 195.35 and a nice decreasing wing between λꙨ 195.44 and λꙨ 195.52. In between we see a rather slightly variable activity. Comparing this graph with that of 1985 and 1998 (figures 5a/b from [9]), a clear peak is visible in both graphs 1985/1998. The graph from 2018 (figures 4 and 6) clearly shows a “capped” ZHR curve without a sharp peak. So here we see the result of the passage of the Earth through the dust trail of 21P in 1985! The activity curve is the sum of the activity of the various disturbed and thus thinned dust trails.

Figures 5a and 5b. ZHR graphs of the Draconid outbursts in 1985 and 1998 from [9]

Figure 6. The same graph as figure 2 of the Draconids ZHR in 2018, but on a logarithmic scaleto compare the ZHR with figures 3a, 3b

This analysis seems to be supported by the observations of the CAMS BeNeLux network. We see a large diffuse radiant here (figure 7). An extensive analysis of the CAMS data may perhaps bring more clarity about the possible dust trails that caused this outburst.

Figure 7. All radiant positions of the Draconids 2018 outburst as recorded by CAMS BeNeLux network

Conclusion
The Draconids showed a nice outburst in 2018. The activity was much higher than predicted. The resulting ZHR curve shows a rather irregular activity of the Draconids during the outburst, as a result of the passage of the Earth through the dust trail in 1985.